High Ionic Conductivity in Oxygen-Deficient Ti-Substituted Sodium Niobates and the Key Role of Structural Features

NaNb1–xTixO3–0.5x (0 < x ≤
0.15) compounds were prepared using a two-step
synthesis process involving a hydrothermal route at T = 200 °C in an autoclave followed by heat treatments under
air or reductive conditions. Rietveld structural refinements from
X-ray diffraction data combined with 23Na and 93Nb nuclear magnetic resonance evidenced the formation of new complex
oxides crystallizing in the P21ma space group. Ti substitution for Nb atoms contributes
to stabilize the acentric polymorph rather than the well-known thermodynamically
stabilized network (Pbma space group) of sodium niobate.
Taking into account the competitive bond sequence Na1(2)–O1(2)–Nb–O3(4),
the large variation of Na1–O1 and Na2–O2 bond lengths
after Ti substitution leads to reduction of the Nb/Ti–O1 and
Nb/Ti–O2 apical distortion (elongation in one direction) and
consequently exaltation of the distortion in the equatorial plane.
Then, transition metal crystal field splitting, as well as the second-order
Jahn–Teller effect, increases, and the optical band gap red-shifts
to visible range starting with low Ti content. Two phase transition
sequences at moderated temperature are characterized by the relaxation
of the perovskite framework with various [Nb­(Ti)­O6] octahedral
distortion and tilt modes: from the polar orthorhombic P21ma phase to the centrosymmetric orthorhombic Cmcm network above T = 300 °C and
then to the ideal cubic perovskite structure above T = 600 °C. The pronounced decrease in phase transition temperature
with Ti substitution, especially from P21ma to Cmcm, was correlated to the
almost identical stabilities of the two Na sites and four oxygen positions
in P21ma symmetry but
also to larger distortion of the transition metal polyhedron enhancing
the oxygen mobility. Moreover, the high ionic conductivity of oxygen-deficient
NaNb1–xTixO3–0.5x was evidenced for the
first time between 300 and 700 °C (σion (T = 300 °C) = 3 × 10–5 S·cm–1 and σion (T = 700
°C) = 2 × 10–3 S·cm–1, for x = 0.15).